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By Andrew J. Streifel, MPH, REHS
Airborne infectious particles are a potential source of hospital infections. Control of airborne microorganisms depends on measures consistent with aseptic technique as well as contamination control connected with ventilation. Human-shed organisms can be controlled using ventilation; however, low infection rates in orthopedic surgery have resulted from contamination control efforts associated with body suit containment.1 Control efforts are effective, but containment of human-source microbes is integral for infectious disease management. The focus for airborne infection transmission is on both the human-source and environmental-opportunist control. The difficulty of monitoring human-source microbes involves complex sampling strategies for recovering the microbes from the air.
The opportunistic environmental microbe, such as a fungal spore, can be assessed using monitoring methods. In hospitals, it is important to control airborne environmental fungal spores because of difficult treatment modalities associated with filamentous fungal infections. The ability to treat diseases that make the patient susceptible to common fungi is becoming more common as well.
Human-source microbes, especially Staphylococcus and Enterococcus species, are controlled with standard contact precautions and not ventilation. Ventilation is critical and can be used to control the spread of airborne human-source infectious agents such as Mycobacterium tuberculosis, Varicella zoster and Rubella. Such airborne-spread microbes are human-source, which develop droplet nuclei after expulsion of droplets from the mouth, especially if the original droplets are less than 150µm in diameter. These droplet nuclei float in air currents and can expose a susceptible host to infection.
Environmental microbes such as Aspergillus or Legionella species can cause opportunistic infection with the airborne transmission of infectious particles. The lung is often the primary site of infection. Indoor fungal organism growth is associated with water-damaged material. For example, sheet rock water-damaged for an extended time (more than three days) by a roof leak provides an excellent growth media for fungi. These loci of fungal growth can be aerosolized and breathed if conditions are ideal. The conditions that cause such release can include construction, renovation, maintenance, open windows, insulated water-damaged ducts, or carpet cleaning.
Air quality indicators for infection control should include viable and non-viable airborne particles analysis. In the case of non-viable particles, measurements can indicate control of airborne particles, especially if methods such as filtration are used to remove these particles from incoming outside air. Internal reservoirs of microbes may become airborne if disturbed and are difficult to differentiate with non-viable samples.
Airborne fungal organisms can be recovered from outside air and are common in relatively high numbers in agricultural regions. Internal sources will result from water damage to organic-based materials, especially modern building materials such as gypsum board or ceiling tile.2 Air sampling can reveal internal sources if air movement or disturbance releases the aerodynamic spore into the air either from outdoor or indoor sources. Air sampling is useful for ensuring ventilation parameters during status quo conditions and especially at the commissioning of a new or renovated system. Spore-forming fungi release most of their spores during deliberate disturbance of loci of growth or spore accumulation. Detection of a release is difficult because sampling after the fact does not correspond to the event, which aerosolized the spores.
Nonviable airborne particles can be detected with the use of a particle counter that allows for real-time air-quality analysis. There are both optical and laser particle counters available. It is important to be able to differentiate different particle sizes. The most useful devices for measuring particle sizes are those that determine particle size diameters greater than 0.5µm, >1.0µm and >5.0µm per cubic foot with each sampling. The particles at >0.5µm are used for assessing a clean room and as a standard utilizes a Military Standard 209(e), which classifies clean rooms with particles per cubic foot that are less than a number. The classification is based on increments of 10. In a HEPA-filtered (99.97% efficient at 0.3µm diameter particles) operating room or bone marrow transplant environment with no people air particles, counts should be capable of Class 1000 clean room status or better.
As a definition for a Class 1000 clean room, "there are less than 1000 particles per cubic foot greater than 0.5µm in diameter" to achieve a clean-room status. This information is especially useful for ensuring filtration integrity or detecting air infiltration in a critical environment before the area is occupied. Particle counters are useful for comparison.
Viable airborne particle analysis is more complex because laboratory expertise is necessary to accomplish this type of detection and analysis. The purpose for sampling should include determination of what the sampling is expected to evaluate. For example, an air sampling search for human-shed microbes such as Mtb or Staph species should not be considered for sampling because of the difficulty in culturing the slow-growing Mtb and for the fact that Staph is a known human microbe that is shed. Aerosols generated by a medical device, such as a drill, may be instructive for air sample evaluation but certainly not routine in any setting. Air sampling should be considered only for evaluating the presence of airborne fungi and determining air quality status.
Evaluation of the air for airborne fungi will yield information that may be helpful in preventing infection or determining the source of airborne opportunistic environmental fungi. Sampling for airborne fungi should be considered in areas where patients are at risk to infections from these opportunistic fungi. The media used for sampling a hospital environment should be clinically relevant. Because the fungi are capable of growth on a variety of media, clinical media such as Sabourauds or Inhibatory Mold agar will provide direct morphological identification from the recovered isolates.
The presence of opportunistic fungi capable of growth at body temperature is of particular concern. Differences between fungi growing at room temperature (25°C) and body temperature (37°C) are generally greater than 90% except in highly filtered environments. The most common fungi in hospital exposure occur from improperly filtered incoming air or from internal sources that were disrupted because of construction or maintenance. Air sampling will not prevent infections during construction. Air sampling can provide information that should inform infection control practitioners that the air quality is good enough for safe patient care because control measures are in place. It is difficult to detect the short-term high-dose exposures that occur because of sporadic environmental disruption.4
A variety of apparatus are capable of viable air sampling.3
A volume of air must be sampled. Settle plates depend on gravity to settle single spores. Opportunistic fungal spores such as A. fumigatus and A. flavus are less than 5.0µm in diameter and represent a buoyant aerodynamic particle. Clumps of particles will settle, but perhaps the most problematic particles are those that are capable of entering the deep lung and are represented by single spores. Collecting the particles in sufficient quantity is essential to detecting low levels of spore concentration causing nosocomial infection. Arnow2 reported infection rates of about 4.0% with A. flavus at 1.1 to 2.2 cfu/m^3. Rhame4 reported 5.0% infections with A. fumigatus at 0.9 cfu/m^3. These low levels do not represent the "spikes" of high levels released during disruptive activity.5 Streifel reported a long-term increase in airborne fungal levels when a water-damaged sink shed organisms into a bone marrow transplant ward.6 The disadvantage of most samplers used in hospitals is represented by low-volume sample capability. Most samplers are designed to sample dirty environments. Samplers that sample 1 cfm may miss capturing of spores at levels under 1.0 cfu/m^3. Hospital air samples should be at least 35 cubic feet or 1.0 m^3 in order to detect low levels.7 Disadvantages of samplers include low-volume sampling, drying of media with long sample times, hard-to-work-with culture media/plates, and difficult-to-calibrate air samplers. A slit-to-agar sampler with times up to 60 minutes may be the best choice of sampler dependent on the sampler's timer, noise levels, and portability.
The timing of sampling is important for detecting airborne fungal levels and for interpreting results. For example, activity evaluation with an air sampler may reveal high concentrations of airborne fungi during renovation activity of a water-damaged bathroom. The best use of air sampling is before occupancy to determine proper filter installation and room pressurization. The purpose of such sampling is to establish rank order for the cleanest areas. The best filtration should demonstrate the lowest particle or viable airborne fungal counts. Such numbers should be considered baseline levels before occupancy. Subsequent sampling should take into account people and conditions, such as incorrect airflow in a protective environment. The exposure to high levels of airborne infectious agents over a short time is probably the greatest risk to the host.5 The ability to capture such events is difficult. Sampling of the environment should be used to determine if the ventilation systems are working; therefore, the areas with the best filtration, pressurization, and air exchanges should have the lowest airborne fungal counts. This result should also be true for non-viable airborne particles detected with a particle counter. Particle count information can be used as a "rule out" during outbreak investigations, especially if appropriate ventilation is installed.
If pathogens (Aspergillus fumigatus, A. flavus, or other opportunists capable of growth at body temperature) are recovered from protected environments, considerations should be taken for single-plate hits vs. multi-plate hits from pathogenic fungi. Random isolate recoveries may be represented by single counts on a plate. Greater than two, for example, Aspergillus fumigatus may represent a point source within the patient care environment. Repeat sampling under such circumstances should determine if it was a passing phenomenon.
Pressure monitoring is important to assure that the protective and airborne infection environments are appropriately pressurized. Pressure relationships drive the airflow. Within the past decade, micromonometer technology has developed to measure air pressure relationships at less than 0.001 inches water gauge (Digital Pressure Gauge, Energy Conservatory, Minneapolis, Minn). Such devices do quantitative testing of special ventilation rooms to determine a numerical pressure relationship rather than a subjective smoke stick evaluation. Smoke sticks provide airflow direction evaluation information useful for investigative analysis of hospital ventilation. Protective room environments should have pressure relationships with pressure differential >0.01 inches water gauge. The airflow should be out of the protected environment (+ pressure).
Conversion Factors and Calculations
The following conversion factors may prove helpful:
1.0 ft^3=28.4 liters
1.0 meters^3=35.3 ft^3
Total colonies per plate should be divided by the volume of air sampled (in liters) to determine colony-forming units(cfus) per cubic meter.
For a list of references, go to www.infectioncontroltoday.com.
Andrew J. Streifel, MPH, REHS, is a hospital environment specialist at the Department of Environmental Health & Safety, University of Minnesota (Minneapolis,Minn).For a complete list of references click here